A short-wavelength, high-power blue laser is expected to be more widely used in an optical pickup or the like. In general, plastic is easily damaged by a laser. Therefore, some optical parts of devices using a short-wavelength, high-power blue laser consist of glass instead of plastic to avoid damage by a laser. For this reason, the prices of devices are relatively high. The relatively high prices are a hurdle to be overcome to enlarge the market of the devices.
At present, plastic materials which can cope with a relatively-low-power blue laser are supplied from various material manufacturers. However, there is no plastic material which can withstand a high-power blue laser.
On the other hand, an antireflection film is frequently formed on surfaces of plastic lenses used in video camcorders, still cameras, glasses, and the like. Such an antireflection film consists of a multi-layered film obtained by alternatively forming low-refractive-index layers and high-refractive-index layers. The multi-layered film is described in Japanese Patent Applications Laid-Open Nos. 11-30703, 11-171596, 11-326634, 2002-71903, 2003-98308, 2003-248102, and the like. However, a conventional antireflection film cannot prevent a damaged caused by a high-power blue laser or ultraviolet light.
Generally an optical element in which a multi-layered film is provided on a plastic substrate cannot be used for light having a wavelength shorten than 300 nm because a transmittance of the light for the plastic substrate is remarkably lowered. On the other hand, the optical element can be used without any difficulty for lights having wavelengths longer than 450 nm because of little deterioration (damage) of the plastic substrate caused by the light. Therefore, there is generated deterioration (damage) of the plastic substrate of the optical element in which the multi-layered film is provided on the plastic substrate becomes troublesome for lights in a wavelength range of 300 nm to 450 nm.
Under the background art mentioned above, there is a need of a method for producing an optical element, in which a multi-layered film is provided on a plastic substrate, having a resistance against lights in a wavelength range of 300 nm to 450 nm.